It is well known in the torque-sensing art that the deformation, or twist, of a rotary shaft under load can be sensed as a measure of the torque being transmitted through the shaft. In relatively low torque applications where the amount of twist may be too small for accurate measurement, such as in automotive power steering systems, the twist is typically augmented by inserting a torsion bar between two relatively rigid sections of the steering column shaft. The relative rotation of the more rigid sections of the shaft may be mechanically or electrically detected using a variety of techniques.
In a safety-critical system such as vehicle steering, any single-point failure must be safe. Torque transducers used in existing vehicle steering-assist systems include a torsion bar that can fracture. If fracture occurs, this can result in an unsafe vehicle control mode. It is an object of the present invention to eliminate this undesirable failure mode by providing a new and improved fail-safe power-assist vehicle steering system.
In the case of the conventional hydraulic power-assist power steering system in common use today, the torsion bar may be in the form of a separate element, as in the case of a conventional rotary hydraulic power steering valve. This hydraulic control valve is actuated by the twisting of the torsion bar. That twisting in turn causes the sleeve valve to open, resulting in power-assist to the steering linkage that is proportional to the torque input exerted on the steering wheel by the vehicle operator. The torsion bar has two functions in this typical hydraulic system: (1) to tune the system dynamics (feel) and (2) to provide the proportional strain that opens the hydraulic control valve for the power-assist system.
However, if the torsion bar breaks, the resulting power-assist applied to the steering linkage is no longer proportional to driver input torque, but can range from full left power-assist to full right power-assist with virtually no change in driver input torque, thus causing vehicle instability even though the shunt is operational to provide a direct mechanical link between the steering wheel and steering gear in parallel with the broken torsion bar in the steering column. There is therefore a need for a fail-safe system for protecting a torque-sensing device from such unstable operation in the event the torsion bar breaks, and which also provides the aforementioned two torsion bar functions.
Generally the use of a torsion bar requires the use of a conventional mechanical shunt fail-safe mechanism, i.e., a torque-limiting device to prevent failure of the torsion bar when unavoidable torque overload conditions occur. Such torque-limiting devices are well known in the art of vehicle steering, and will therefore not be described in this specification. Instead, such devices will be merely referred to as “shunts”.
In general, and by way of summary description and not by way of limitation, the present invention overcomes the aforementioned torsion bar failure problem by providing a torque transducer that includes first and second torsion bar stages connected in series. The first stage is constructed to have a lower torsion spring rate (i.e., force-constant or stiffness coefficient) than the second stage. A conventional torque sensor is operatively coupled only to the second stage for measuring applied steering torque as a function of torsional strain produced by the torsional stress transmitted therealong. Only this second stage and associated torque sensor are used to provide the torque-measuring signal in the system.
The first torsion bar stage is constructed to have a failure mode at a lower stress level than the second stage torsion bar stage, and thus is provided as a sacrificial “weak link” in the torque transducer. Therefore, if a stress related failure ever occurs, it will occur only in the first stage. Such failure in the first stage in turn will prevent any torque from reaching the second stage torsion bar. With zero input torque to the second stage upon first stage failure, the torque sensor will see only zero strain, thereby providing a system that is fail-safe because only a “zero” torque signal can be generated in the event of torsion bar failure, thereby avoiding the aforementioned vehicle instability problem. Moreover, in normal operation, the first stage torsion bar may be used to provide the low torsion rate for tuning of the steering system, i.e., to tune by initial design the system dynamics or “feel” and response for the steering system.